1. Field of the Invention
The present invention relates to an apparatus for over current detection of a dual motor for use in a vehicle. More particularly, this invention is directed to an over current detection apparatus of a dual motor in a vehicle which is capable of preventing each motor from burning out by more precisely detecting an over current of each motor in the dual motor within an overall drive area thereof, wherein this is achieved by setting, as an over current detection reference current, a predefined range between a normal current and a constraint current of each motor by drive input conditions of each motor, detecting a current of each motor flowing in a field effect transistor (FET) via a shunt resistor and a differential amplifier, and comparing the detected currents with the over current detection reference currents.
2. Description of Related Art
In general, a dual motor is known to provide a driving force necessary for vehicle operation, wherein a power required by the vehicle is supplied by changing a rotary power under the control of a microprocessor. Such a dual motor may burn out due to over current that occurs due to internal, external, or constraint effects, etc., during the operation of the motor.
As such, since the prior art dual motor burns out due to over current unless such over current is accurately detected and the operation of the dual motor is stopped to prevent burn out upon the detection of the over current, an over current detection of the dual motor has conventionally been performed through an over current detection device as illustrated in FIG. 1, in order to prevent the above phenomenon.
FIG. 1 is a circuit diagram showing a configuration of an over current detection device of a dual motor used in a vehicle according to the prior art. This prior art device comprises an input unit 1 for inputting an external signal including motor drive information with duty cycle, a microprocessor 2 that provides an FET drive control signal to drive an FET according to the motor drive information in the external signal from the input unit 1, determines whether or not there occurs an over current state in the dual motor based on an output from a comparison unit 9 to be described below, and controls the dual motor to halt its operation upon the occurrence of the over current, an FET driver 3 for generating an FET drive signal in response to the FET drive control signal provided by the microprocessor 2, an FET 4 as a dual motor driving element that is turned ON/OFF depending on the FET drive from the FET driver 3 to drive the dual motor simultaneously, a freewheeling diode 5 for passing a recovery current for continuous operation of the dual motor when the FET 4 is OFF, a first and a second motors 6 and 6′ as the dual motor that are driven according to the operations of the FET 4 and the freewheeling diode 5 and provide a driving force necessary for the vehicle's operation, a voltage detector 7 for detecting a voltage that is taken between both ends of the FET 4 produced by a current flowing in an internal resistor of the FET 4, a reference voltage setting unit 8 for setting an over current detection voltage for the first and the second motors 6 and 6′ as a reference voltage, and the comparison unit 9 for comparing the voltage detected by the voltage detector 7 with the reference voltage set by the reference voltage setting unit 8 and providing the microprocessor 2 with an output voltage of high level implying the normal state if the detected voltage is less than the reference voltage and with an output voltage of low level meaning the issuance of the over current state if the detected voltage is above the reference voltage.
FIGS. 2A and 2B show output waveforms resulting when the dual motor is in the normal state and over current state, respectively, according to the conventional over current detection device. With reference to these drawings, an operation of the conventional over current detection device will be described in detail below.
At first, the microprocessor 2 in the prior art over current detection device inputs the motor drive information included in the external signal from the input unit 1 and controls the FET driver 3 according to the motor drive information in order to drive the FET 4 as the dual motor driving element. Upon the ON operation of the FET 4, the first and the second motors 6 and 6′ are driven by a power (or supply voltage); and upon the OFF operation of the FET 4, the recovery currents issuing from the first and the second motors 6 and 6′ enable them to be continuously driven by passing those currents flowed through the freewheeling diode 5.
The FET 4 includes an internal resistor and the ON operation of the FET 4 is controlled according to the motor drive information. Based on the current variation by such ON operation of the FET 4, the voltage taken between both ends of the internal resistor in the FET 4 is varied.
After that, a comparator contained in the comparison unit 9 compares the voltage between both ends of the FET 4 detected by the voltage detector 7 with the reference voltage provided by the reference voltage setting unit 8. From the comparison, the comparator provides the microprocessor 2 with an output voltage of 5 V if the detected voltage is less than the reference voltage, and with an output voltage of 0 V if not, i.e., if the detected voltage is above the reference voltage.
Then, the microprocessor 2 determines that the first and the second motors 6 and 6′ are in the normal state if the output voltage from the comparison unit 9 is at a high level of 5 V; and conversely, it determines that an over current state is present in any one or all of the first and the second motors 6 and 6′ if the output voltage is at a low level of 0 V, and halts the operation of those motors, in order to prevent the motors from burning out due to the occurrence of the over current.
In the conventional over current detection device of the dual motor for use in the vehicle as mentioned above, however, there exist some problems as set forth below.
That is, since the internal resistor in the FET exhibits slight variation due to manufacturing tolerances and thus is varied depending upon the temperature for the same FET, a dispersion of the detected voltage by the current flowing in the FET becomes large.
Consequently, there may be a problem that the power necessary for the vehicle's operation is not supplied thereto due to the interruption of the dual motor that arises when the detected voltage exceeds the reference voltage upon their comparison, even under the normal state. In particular, in the case of an engine cooling motor, the engine may overheat, causing the vehicle to break down.
Further, under the condition that an over current may occur, if the dual motor is driven continuously as the detected voltage is less than the reference voltage to thereby make an issuance of a larger current, there ultimately exists an instance where the dual motor burns out.
In addition, since the reference voltage set by the reference voltage setting unit for the over current detection cannot be modified, it may be determined that the detected voltage is less than the reference voltage, upon an issuance of the motor over current when the motor is driven with low current. Accordingly, over current of the dual motor is not detected, leading to motor burn out.
Especially, in case the dual motor is simultaneously driven, when one motor with low capacity is in constraint state, a current of one motor is combined with that of the other motor with large capacity. In such a case, in the range below the maximum current for the normal drive of the motor with the large capacity, it is difficult to sense the over current state of the motor with the low capacity, which results in burn out of that motor.